As known, an optical cable typically comprises an optical core including one or more optical fibers and an external sheath enclosing the optical core. The external sheath is typically made of a polymeric material and has the primary function of protecting the optical core from the mechanical point of view.
Within the external sheath, the optical fibers may be arranged in various ways. In particular, in the so-called “loose tube cables”, the optical fibers are loosely arranged within one or more buffer tubes. Each buffer tube typically contains multiple fibers, and the individual fibers are free to move relative to one another within the buffer tube. In the so-called “central loose tube cables” (briefly, CLT cables), all the optical fibers of the cable are loosely arranged within a single buffer tube, which is in turn enclosed by the external sheath. In the so-called “multi loose tube cables” (briefly, MLT cables), the optical fibers are instead divided into multiple bundles (e.g. 3, 4 or 6 bundles), the optical fibers of each bundle being loosely arranged within a respective buffer tube so as to form an optical unit. The buffer tubes are then stranded according to an open helix or S-Z arrangement, typically about a central strength member. A binder may also be provided around the buffer tubes for retaining them. Both in CLT cables and in MTL cables, the external sheath may comprise two side strength members (typically made of steel or fiber reinforced resin) embedded within the sheath's thickness and placed at diametrically opposed positions.
Loose tube cables are typically used for applications where the optical fibers must be individually extracted from the cable and spliced, e.g. in FTTH and FTTP applications. For instance, drop cables of FTTH or FTTP networks are typically implemented as CLT or MLT cables with a particularly reduced diameter (less than 10 mm).
In FTTH or FTTP networks (and more generally in terrestrial networks), several cables are typically laid down within a same duct, for instance an underground duct in case of metro networks or a vertical duct fixed to or embedded within a wall of a building, in case of FTTH or FTTP application.
For allowing identification of the various cables laid down within the same duct, it is known providing the various cables with respective identification codes printed on the external surface of their sheaths. The codes are typically printed by an ink-jet printing technique.
U.S. Pat. No. 6,347,172 discloses a fiber optical communication cable comprising an optical fiber, a PVC buffer, an aramid strength layer and a cable jacket. According to one embodiment, a side emitting optical fiber is embedded within (or positioned underneath) a translucent section of the jacket, which allows the side emitting optical fiber to be observed therethrough. The remaining portion of the jacket is made of an opaque material. Alternatively, all the cable jacket is made of translucent material. According to other embodiments, the side emitting optical fiber is located at an outer perimeter of the jacket, embedded within an additional cable jacket, which is the cable outermost layer. The translucent jacketing material may be tinted with or without shading pigment and allows the side emitting fiber to be seen when illuminated with a dedicated light source. Additionally, the side emitting fibers may be clear or tinted with color (e.g. violet). When a detecting light is put into the ends of a number of cables, the individual cable emitting a violet light along its axis may be detected amongst the various other cables (i.e. red, blue, clear, green, etc.).